Sunday, April 22, 2007

Nasty, brutal, short - the life of most humans

I recently wrote of 19th century American aspirations for a 50 year lifespan. John Hawks describes other settings when life expectancy was about 35 ...
John Hawks Anthropology Weblog : 2007 03

Kim Hill and colleagues (2007) report in the current Journal of Human Evolution on the mortality profile of recent Hiwi hunter-gatherers. Here is their abstract:

"To our knowledge, interview-based life tables for recent hunter-gatherers are published for only four societies (Ache, Agta, Hadza, and Ju/'hoansi). Here, we present mortality data for a fifth group, the Hiwi hunter-gatherers of Venezuela. The results show comparatively high death rates among the Hiwi and highlight differences in mortality rates among hunter-gatherer societies. The high levels of conspecific violence and adult mortality in the Hiwi may better represent Paleolithic human demographics than do the lower, disease-based death rates reported in the most frequently cited forager studies."

...Among pre-1960 Hiwi males, 57 percent could expect to survive to age 15, and 43 percent to age 30, with an average young adult mortality rate of around 2 percent annually. So it is not anything like as high as has been suggested for Neandertals and earlier humans (with annual mortality rates as high as 6 percent).

The most interesting aspects of the paper are the comparisons between the Hiwi and other ethnographically-known hunter-gatherers. Many of the differences in mortality profiles are attributable to strong cultural differences:

"Cause of death among the groups differs considerably. Disease is an important cause of death in all groups, but represents only 20% of deaths in the precontact Ache, 45% among the precontact Hiwi, and about 75-85% of all Hadza, !Kung, and Agta deaths. Respiratory disease is the main killer of the Ache, whereas gastrointestinal pathogens are most important among the Hiwi and probably Hadza. Among the !Kung, respiratory and gut infections are about equally important. Violence is the major cause of death among the precontact Ache (55% of all deaths) and very important among the Hiwi (30% of all deaths), but notably less important in the two African societies and the Agta (3-7% of all deaths). Indeed, the crude homicide/warfare death rates per year lived are more than ten times higher among the Hiwi and Ache than among the Hadza or !Kung (1/100 and 1/200 per year for precontact Hiwi and Ache, respectively, vs. 1/2500 and 1/3000 for the Hadza and !Kung, respectively)..."

"If high mortality, warfare, homicide, and accidental trauma are typical of our Paleolithic ancestors, the Hiwi mortality patterns may be more representative of the past than those derived from other modern hunter-gatherers. If so, several observations about the Hiwi are important. First, conspecific violence was a prominent part of the demographic profile, accounting for many deaths in all age and sex categories. Most of the adult killings were due to either competition over women, reprisals by jealous husbands (on both their wives and their wives' lovers), or reprisals for past killings. The criollo-caused killings were motivated by territorial conquest. Moreover, infanticide (especially on females) constituted the highest mortality rate component of all Hiwi conspecific violence. Second, no predation deaths were reported despite attacks by anacondas, Orinoco caimans, and piranhas, and the presence of jaguars in the area. Accidents associated with the active-forager lifestyle were common, but disease was a more important killer, accounting for nearly half of all deaths. This suggests an adaptive landscape in which success in social relations, competitive violence, and disease resistance are paramount. This may partially explain why many of the genes that appear to have been under strong selection in the past 50,000 years affect either disease resistance or cognitive function (Wang et al., 2006), presumably related to success in an atmosphere of frequent violent social competition (Hill et al. 2007:451)."
These communities vary greatly in their level of violence. The most violent of them sound comparable to chimpanzees. The big surprise is how dominant these pre-technological human predators are. They died of disease or from conspecific violence or from accidents, not from other predators.

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